The present invention relates to a multilayer headbox.
More particularly, the invention relates to a multilayer headbox of the type having a slice chamber and in the slice chamber a rigid separator vane for keeping stock flow streams on each side of the vane separated from each other, said slice chamber having a downstream portion converging in the direction of the stock flow and ending in a slice opening, said vane having an upstream end and a square downstream end, said vane being securely fixed in cantilever fashion at said upstream end and having its downstream end unattached and free, said vane being sufficiently rigid to be capable of supporting unequal pressures and velocities in the stock flow streams, said headbox further having a vane extension having an upstream end and a downstream end, the upstream end of the vane extension being thinner than and exchangeably anchored to the square downstream end of the separator vane to form an extended vane assembly having a step on each side of the assembly, the downstream end of the vane extension being unattached and free and located downstream of the slice opening.
Such a multilayer headbox is disclosed in Canadian Patent No. 1,139,142 (AB Karlstads Mekaniska Werkstad). In this headbox, widely known as the KMW Air Wedge Headbox, the rigid vane (or vanes) may consist of a glass fiber reinforced epoxy resin and have a constant thickness of 12 millimeters (about 1/2 in), for example. The vane has internal channels for supplying air to its downstream edge, which is located slightly downstream of the slice opening. Thereby, there is formed at the downstream edge a wedge of air that keeps the stock flow streams on each side of the vane separated part of a distance to the forming zone of the papermaking machine, while the stock flow streams travel through surrounding air. A vane extension formed by a comparatively thin flexible foil may be exchangeably anchored to the square downstream end of the vane to keep the stock flow streams separated a further part of the distance downstream of the edge of the air wedge. Such a foil will eliminate any velocity components perpendicular to the stock flow streams and thereby contribute to an improvement of the layer purity and the layer formation.
FIGS. 9b and 9d and pages 15 to 17 of Canadian Patent No. 1,134,658 (AB Karlstads Mekaniska Werkstad) disclose a design for exchangeably anchoring a foil to a square downstream end of a separator vane. The foil has a row of equidistantly spaced dowels at but spaced from its upstream end. The dowels are of a larger length than diameter, and all of the dowels extend through the foil and project equal distances in opposite directions from the foil. A longitudinally extending groove for receiving the upstream end of the foil including the dowels is provided in an end face of the square downstream end of the vane. Both sidewalls of the groove have a longitudinally extending recess for accommodating the projecting parts of the dowels. The groove is placed symmetrically in the end face, so that the steps formed on both sides of the vane-foil assembly are equal.
As disclosed in U.S. Pat. No. 4,436,587 (Andersson), multilayer paper of superior layer purity and layer formation can be produced by discharging a plurality of superimposed jets of papermaking stock from an air wedge headbox into the throat of a roll type twin wire former, and maintaining the velocity of the jet closest to a plain forming roll in the roll former slightly higher than the velocity of an adjacent discharged jet. The separator vane or vanes provided in the slice chamber are sufficiently rigid to be capable of supporting unequal pressures and velocities in the stock flow streams. By controlling the pressure in one stock flow stream relative to the pressure in an adjacent stock flow stream, a pressure difference across the vane may be created. This pressure difference causes a deflection of the vane, which results in a movement of the downstream end of the vane, so that different jet velocities are produced while the flow rates remain constant.
The air wedge multilayer headbox has been on the market for over a decade. Its most pronounced advantages have been its ability to produce an excellent layer purity and the durability of its separator vanes. The experienced life is several years. However, one or two 12 millimeters (about 1/2 in) thick vanes extending out of the slice opening means that the total slice opening, that is slice lip to slice lip, has to be large and, consequently, a long free jet from the slice opening to the forming zone is required. Even though the two or three jets, one for each layer in the paper to be produced, are kept separated from one another by the air wedges and the possible foils for a considerable portion or even all of the distance to the forming zone, the cross sectional shape of the jet deteriorates with the length travelled by the free jet. Thus, a layer formation of the same excellent class as the layer purity can not be achieved. In addition, the flexible foils risk being damaged on an exchange of forming fabrics.
U.S. Pat. No. 4,812,209 (Kinzler et al.) discloses another type of multilayer headbox. As in the air wedge headbox, a separator vane extends through the slice chamber from one side wall to the other and through the slice opening to form an upper flow channel and a bottom flow channel and keep stock flow streams separated from each other. However, the separator vane is of a wedge-shaped cross section and has an upstream body portion, which may be of steel and be rigidly connected to an upstream tube bank by means of welding, and a downstream tip portion, which to facilitate exchange may be made of a reinforced synthetic material, as rigid as possible. There is no step at the connection between the body portion and the tip portion of the vane, so the taper of the vane thickness is continuous to the very edge of the tip portion. Instead, the connection is stated to be rigid and at the same time so tightly sealed along the joint that a clinging of fibers is ruled out. Further, each of the headbox side walls is divided into a lower wall section and an upper wall section, which laterally confine the bottom flow channel and the upper flow channel, respectively. The width of the tapered separator vane in the cross machine direction is larger than the distance between the headbox side walls to permit the lateral edges of the vane to be clamped between the upper and the lower wall section on both sides of the headbox.
As a result of the clamping of the lateral edges of the vane, the headbox is unsuitable for operating with unequal pressures and velocities in the stock flow streams, at least in machines that are wider than the very narrowest production machines, because when a laterally clamped vane is exposed to unequal pressures in the two adjacent stock flow channels, the clamping prevents the vane from deflecting ideally and assume a deflection profile, where the vane is straight from headbox side wall to headbox side wall but curved from its upstream edge to its downstream edge. When the vane, which is rigidly connected at its upstream end and clamped along its lateral sides, is exposed to different pressures in the two adjacent stock flow channels, it will assume a slight partially dome-shaped deflection profile. The profile from side wall to side wall will be straight at the upstream edge of the vane but become more curved with increasing distance from the upstream edge, and at both of the side walls the profile from the upstream edge to the downstream edge will be straight but become more curved with increasing distance from the side walls. Consequently, since the downstream edge of the vane will not remain straight, the layer caliper and/or the layer basis weight profile will vary over the width of the produced web.